Photo-sensitive imagers collect charge due to incident light to sense the presence or absence of objects which affect the amount of incident light available to the imager. In the automotive industry, for example, imagers are used as sensors in lane tracking systems, edge-of-road detection systems, near-obstacle-avoidance detection systems, crash alert systems and passenger position sensing systems. Such safety critical applications require imagers with high reliability. Accordingly, conventional imagers, which are typically photo-diode imagers, are tested at the manufacturer using precision equipment to inject a known quantity of light and compare the imager output to the expected output resulting from the known quantity of light. Such test equipment is generally quite sophisticated and correspondingly expensive. Additionally, operational testing at the manufacturer, while beneficial for a variety of reasons, does nothing to address the need for monitoring imager operation and detecting in-field failures or changes in imager performance.